Aqueous emulsion type pressure-sensitive adhesives are extensively used in recent years in place of organic solvent-based pressure-sensitive adhesives from the standpoints of the hygiene of working atmosphere, etc.
However, the aqueous emulsion type pressure-sensitive adhesives generally are inferior in water resistance to solvent-based pressure-sensitive adhesives and have had a problem that the pressure-sensitive adhesive layer formed therefrom is apt to blush due to moisture absorption when exposed to water or moisture. Elimination of this problem of blushing has been desired especially in applications where a pressure-sensitive adhesive sheet using a transparent substrate is used or where transparent materials are bonded to each other with a pressure-sensitive adhesive.
Known techniques for inhibiting the blushing of an aqueous emulsion type pressure-sensitive adhesive include a method in which an aqueous emulsion wherein the dispersed particles have a reduced diameter is used, a method in which a pressure-sensitive adhesive having a heightened crosslink density is used, and a method in which the use amount of a hydrophilic compound such as a surfactant is reduced. For example, such pressure-sensitive adhesives which have been proposed include: a pressure-sensitive adhesive composition comprising a colloidal acrylic emulsion having a particle diameter of from 0.001 to 0.009 .mu.m and a crosslinking agent (see JP-A-62-297372; the term "JP-A" as used herein means an "unexamined published Japanese patent application"); a pressure-sensitive adhesive composition comprising as an essential component an aqueous emulsion having an average particle diameter of 100 nm (0.1 .mu.m) or smaller and a crosslinked structure (see JP-A-1-170677); and a pressure-sensitive adhesive composition containing an acrylic copolymer having an average particle diameter of 0.3 .mu.m or smaller and a gel content of 40% by weight or higher (JP-A-7-26229).
However, the techniques disclosed in the above publication references are not fully effective in inhibiting the blushing of the aqueous emulsion type pressure-sensitive adhesives. A further improvement is still desired.